The present invention relates to an improved directional microphone system. In the prior art, various microphone designs are known including those which have an omnidirectional beam pattern and those having a unidirectional (cardioid) beam pattern. Such well known microphones have been known to be used in voice recognition systems and public announcement (PA) systems utilizing some noise cancellation techniques. While several different noise cancellation techniques are known in the prior art, none of these techniques is believed to be closely related to the teachings of the present invention.
The following prior art is known to Applicant:
U.S. Pat. No. 3,644,674 to Mitchell, et al. discloses an ambient noise suppressor including four microphones. The microphones are set up so that each one is the same distance from the talker. Since the talker is at equal distance from all microphones, any sounds which emanate from a source which are not equidistant from all microphones are cancelled out by the circuitry of the system. This is different from the teachings of the present invention since firstly, in the present invention, the speaker is not required to be located at equal distance from each of the microphones. Additionally, the present invention utilizes directional microphones which are not believed to be taught or suggested by Mitchell, et al. Other differences exist, such as the failure of this prior art reference to consider separate frequency components as the present invention does. More importantly, the Mitchell, et al. system does not totally cancel the noise source but only improves the signal-to-noise ratio without the use of a microprocessor whereas the present invention utilizes a microprocessor to cancel the noise rather than merely improving the signal-to-noise ratio.
U.S. Pat. No. 4,008,439 to Schroeder discloses a device for processing two noise contaminated substantially identical signals to improve the signal-to-noise ratio. As disclosed therein, the device includes a pair of input ports into which two noise contaminated substantially identical signals are inputted whereupon a cancellation technique is utilized to cancel noise. Again, the user of the system must be placed at equal distance from each of the microphones in order to produce substantially identical signals. Alternatively, the equal distance can also be adjusted by delay in one of the channels. The system uses weighting factors which continually change which is different from the teachings of the present invention. Further, many of the differences as set forth hereinabove regarding Mitchell, et al. are equally applicable here.
U.S. Pat. No. 4,155,041 to Burns, et al. discloses a system for reducing noise transients in identical electrical carrier signals. The signals are carried on separate channels and the device includes means for comparing the noise transients which occur in each channel and cause a carrier signal to be transmitted into the channel having the lowest noise transients. If the noise transient level exceeds a predetermined amount, the device further includes means for blanking out the excess noise. In the operation of this invention, a switching and a blanking step are sequentially employed. The present invention is believed to be distinct from the teachings of Burns, et al. because firstly , the Burns, et al. system does not use microphones nor does it pick up sound sources from a room. The system on the other hand is used for reproduction of already recorded sound signals which are fed into two separate channels whereupon further processing is made to eliminate extraneous noises such as, for example scratches in a record. Further, many of the differences cited with regard to the Mitchell, et al. invention are also equally applicable here. This discussion is also believed applicable to a further known U.S. Pat. No. 4,359,742 also to Burns, et al.
U.S. Pat. No. 4,420,655 to Suzuki discloses a system including two microphones which respectively receive signals and convey them to respective low pass filters whereupon a differential amplifier receives the filtered signals and then relays them to a further differential amplifier. The inventor claims to affect a compensation for the proximity effect. This system is distinct from the system of the present invention since in the present invention the user is restricted in the location of the microphones to be far enough away from the microphone so that in theory no proximity effect occurs. If desired, the present invention can also be operated so close to one of the microphones that a cancellation of the proximity effect must be made by a technique similar to the one described in this patent. Additionally, many of the differences set forth hereinabove regarding Mitchell, et al. are equally applicable herein.
U.S. Pat. No. 4,485,484 to Flanagan discloses a directional microphone system which arranges microphones so as to focus on a prescribed volume in a large room such as an auditorium. As disclosed, the system is designed to only accept signals which emanate from the prescribed volume and to reject any signals which are received from outside the prescribed volume. The system utilizes two microphone arrays wherein the first array is placed along a first wall and the second array is placed along a second wall or placed on the first wall spaced a predetermined distance from the first array. A separate position locator is employed which determines the position of the speaker. This invention is not ideal because due to phase interferences between the beam processed signals that occur if the microphone arrays are not equidistant from the talker location, resulting signals are not uniform in sensitivity for all points within the desired focal volume. The device further includes a signal adjuster which is adapted, to alter the phase characteristics of the signal applied thereto so that the terms B.sub.a and B.sub.b are equal. For the signal adjuster, a phase vocoder is used. The present invention is distinct from this system since the patented system uses a separate position locator whereas the present invention does not need to find the position of the speaker to effectively cancel out extraneous noises. Further, the patented system requires use of the microphone arrays including many microphones in each array whereas the present invention only uses two or three directional microphones. Additionally, the patented system uses dynamically controlled beams which result in dynamic changes in the received volume. Furthermore, the microprocessor is only used in the beam steering control and everything disclosed in this patent concerning the position locating, the dynamic beam steering (with lookup tables in memory), the microprocessor and the adjustment circuitry is unrelated to the teachings of the present invention.
U.S. Pat. No. 4,066,842 to Allen discloses a system utilizing an arrangement for reducing the effects of room reverberating and noise pickup in which signals from a pair of omnidirectional microphones are manipulated to develop a single less reverberant signal. This is accomplished by partitioning each microphone signal into preselected frequency components, co-phasing corresponding frequency components, adding the co-phased frequency component signals and attenuating these co-phased frequency components signals that are poorly correlated between the microphones. This system is vastly different from the teachings of the present invention for many reasons including the fact that the patented system uses omnidirectional microphones whereas the present invention utilizes directional microphones, the patented system looks at frequency components but only adjusts for phase differences, and further, in light of the same differences from the present invention as were evident regarding the above described Mitchell, et al. patent.
U.S. Pat. No. 4,131,760 to Coker discloses a system for determining the phase differences between the direct path signals of two microphones which system operatively aligns the two microphone signals to form a deverberated signal. Since this system only phase aligns the two microphone signals, it is believed to be of only general interest concerning the teachings of the present invention.
U.S. Pat. No. 2,736,771 to R. L. Hanson, et al. discloses a distant-talking telephone system where a pair of microphones are disposed side by side and facing the speaker in their normal position. After any desired amount of amplification, the outputs of the two incoming signals from the two microphones are added together, averaged and then passed through any desired transmission apparatus to outgoing telephone lines. A difference output is also derived, the individual outputs being preferably clipped beforehand. The difference output is applied as a control signal to vary the gain in the summed output transmission path. This system is vastly different from the teachings of the present invention for many reasons, including the fact that the patented system only averages the two incoming signals and uses the difference between the two incoming signals to control the averaging. Further, the same differences between the present invention and the patent to Mitchell, et al. as described above, apply here.
Applicant is also aware of several non-patent publications which are also believed to be of only general interest concerning the teachings of the present invention. A first category includes those which teach the use of multiple sensors in order to enhance the immunity of speech input to acoustic background noise. A two sensor configuration involving an accelerometer and a gradient microphone is described. A second configuration using one microphone for low frequencies and a second microphone for high frequencies is also described. These sensors are placed within 5 cm of the user's mouth. The system is only to be used with speech input. The systems are vastly different from the present invention since they do not look at frequency components and do not use any area cancellation scheme. Thus, the following publications are believed to be of only general interest concerning the teachings of the present invention.
"Multisensor Speech Input for Enhanced Immunity to Acoustic Background Noise", by V. R. Viswanathan, et al. presented at the International Conferences on Acoustics, Speech and Signal Processing, March 19-21, 1984, San Diego, Calif.;
"Noise-immune Speech Transduction Using Multiple Sensors", by V. R. Viswanathan, et al. presented at the International Conference of Acoustics, Speech and Signal Processing, March 26-29, 1985, Tampa, Fl.
Another catogory of publications includes those which teach specific details of directional microphones per se without disclosing details of systems using such directional microphones. Thus, a publication entitled "On the Use of Directional Microphones for Turbine Generator Sound Level Measurements," by A. P. Hribar, et al. IEEE Transactions on Power Apparatus and Systems, Volume PAS-98, #3, May/June, 1979, and publication entitled "The Quest for Directional Microphones at RCA," by H. F. Olson, Journal of the Audio Engineering Society, Volume 28, No. 11, Nov. 1980 and "Conference Microphone with Adjustable Directivity," by J. L. Flanagan, et al., Journal of Acoustical Society of America Volume 77, No 3, May 1985, are believed to be of only general interest concerning the present invention.
Another group of publications are those which disclose systems including circuitry for noise cancellation which systems utilize only one microphone. These systems are characterized by the use of predictive means for cancelling noise rather than means for measuring actual noise. Thus, the following publications are believed to be of only general interest concerning the teachings of the present invention:
"Adaptive Digital Techniques for Audio Noise Cancellation" by James E. Paul, IEEE Circuits and System Magazine, Volume CAS-1, No. 4, 1979;
"Frequency Domain Adaptive Noise Cancellation in Speech Signal" by Juan Carlos Ogue, et al. Technology Reports, Tohocu University, Volume 48, No. 2, 1983;
"Adaptive Noise Cancellation for a Class of Non-linear, Dynamic Reference Channels" by John C. Stapleton, et al., IEEE. Transactions on Circuits and Systems, Volume CAS-32, No. 2 Feb. 1985.
A further class of publications, one of which is known to Applicant, discloses a system for suppression of acoustic noise in speech utilizing two microphones wherein one of the microphones is exposed to the signal and noise whereas the other microphone is exposed only to noise, and noise compensation is thereby accomplished. The following publication, which is accordingly believed to be of only general interest regarding the teachings of the present invention, is known to Applicant:
"Suppression of Acoustic Noise in Speech Using Two Microphones Adaptive Noise Cancellation" by Steven F. Boll, et al., IEEE Transactions on Acoustics, Speech and Signal Processing, Volume ASSP-28, No. 6, December, 1980.
Thus, a need has developed for an improved directional microphone system which may utilize two or three specially designed directional microphones and which further includes circuitry specifically designed to cancel noise within a predetermined work area by actually measuring the noise and eliminating it. From this perspective, the present invention was developed.